1. Field of the Invention
This invention relates, in general, to programmable time-registering meters for measuring AC electric energy consumption during time differentiated billing rate periods and for controlling a load. More specifically, this invention randomizes the timing of load shedding and reconnecting.
2. Description of the Prior Art
Electric energy billing meters of the electromechanical type are commonly used to measure the amount of electrical energy consumed at the premises of electric energy users. The electric energy supplier bills each user on the basis of the quantity of electric energy consumed during the billing period. Most billing meters in use today measure the total consumption of electric energy (kilowatt-hours) or a maximum peak demand (kilowatt demand) during a given time interval, usually a month. It is often desired to provide an electric energy meter for measuring both kilowatt-hour consumption and kilowatt demand and for separately totalizing these parameters as they are measured during different designated time intervals of each day. This method of metering is known as time-of-day or time-registering metering by those skilled in the art. The time-of-day intervals, typically three, generally reflect those times of the day when electric energy demand is at its highest, intermediate, or lowest level. The billing rates corresponding to these time intervals are usually designated as the on-peak rate, the mid-peak rate, and the base rate, respectively.
Induction watthour meters equipped with mechanical kilowatt-hour registers for measuring consumption during various time intervals of each day are known in the prior art. Selective operation of these registers is provided in response to mechanical time switch mechanisms. However, these mechanical registers and time mechanisms have limited measuring and accumulating capabilities with respect to daily time intervals and variations of these intervals between week days and weekend days. The recent application of solid-state electronic devices to time-registering meters has eliminated the need for mechanical registers and permitted an increase in the number of electrical energy parameters that can be measured. The result has been an increase in the number of rate selection schedules avilable to the utility and greater flexibility in their assignement.
One such solid-state time-of-day meter is disclosed in U.S. Pat. No. 4,283,772 which issued to Johnston on Aug. 11, 1981. This patent discloses a programmable time-registering AC electric energy meter including solid state logic circuitry with a programmed sequence of operation. The meter disclosed also provides for shedding and reconnecting a load at designated times of each day. Note this meter does not provide randomized load control and may therefore cause demand peaks due to simultaneous reconnection of many loads. These demand peaks could be lessened by providing a random time at which each load is to be shed and reconnected.
One common randomized load control method employs a resistor-capacitor timing circuit. A capacitor charges through a resistor, and a subsequent shed command causes the load to be shed for a time determined by the voltage across the capacitor. Variations in the values of the resistor and capacitor cause the loads to be reconnected to the system at varying times. Despite these variations, it has been found that use of RC timing circuits causes clusters of loads to be reconnected to the system within a very short interval. The result is a high differential power demand placed on the electrical energy system.
U.S. Pat. No. 4,213,058 discloses a technique for randomly reconnecting the load to an electrical energy system. When power consumption is excessive a central controller transmits a load shed command to various load management terminals. Upon receipt of the shed command, a load management terminal generates a reset pulse which sheds the load associated with that load management terminal and resets a binary counter. Resetting the binary counter enables the counter to then begin counting asynchronous clock pulses which are being continuously generated within the load management terminal; the clock signal has a period T. The positive going transition of the second clock pulse received by the binary counter after it has been reset inhibits receipt of further clock pulses and causes the load to be reconnected to the power system. As can be seen, the randomness associated with this timing cirucit is due to the uncertainty of whether the reset pulse will be received during a clock pulse or during the space between clock pulses. If the reset pulse should occur immediately before the positive going transition of a clock pulse, the load will be disconnected from the system for a time equal to the period of the clock pulse signal, i.e. T. If the reset pulse occurs immediately after the negative going transition of a clock pulse, the load will be disconnected for a time equal to 2.times.T.
Another method for randomly controlling power system loads is illustrated by the load management terminal of U.S. Pat. No. 4,402,059 which issued to Kennon et al. on Aug. 30, 1983. When the load management terminal sheds a load, it calculates a random number and adds this random number to a predetermined shed time. The sum of these two numbers is loaded into a load control timer. The load control timer is counted down, and when it reaches zero the load is reconnected to the power system.